The present invention concerns a photostimulable phosphor-coated film and a method of testing a digital device for scanning such film with respect to various system parameters.
Digital apparatus is being increasingly employed in medical diagnostic radiography. Photostimulable imaging plates are being employed with increasing frequency instead of conventional X-ray film for X-ray imaging. One advantage of such an imaging plate over conventional film is its almost linear reproduction characteristic. An imaging plate is accordingly much more sensitive to too low or too high doses of radiation. Another advantage of imaging plates is that they can be used over and over; that is, images can be erased and others exposed onto the same plate. Still another advantage of an imaging plate is that it can be handled very much like conventional X-ray film. Accessories, such as cassettes for example, that are already employed for loading and unloading X-ray film can extensively be exploited to load imaging plates as well.
One method of handling photostimulable phosphor-coated film in X-ray cassettes is known, for example, from the published U.S. Pat. No. 4,893,011. In this case, an X-ray cassette is loaded with an imaging plate. Information relating to the particular image, such as patient name, date of birth, physician of record, etc., is entered in a semiconductor memory on the cassette at a data-entry station. The X-ray exposure is carried out and a latent image stored in the imaging plate. The cassette is then inserted in a reader wherein the cassette is opened and the plate is removed and stimulated with a digital scanning device. The light emitted by the stimulated elements, such as a phosphor or fluorine, is intercepted by a collector and converted into digital signals. The signals can then be processed in a digital processor.
The present invention concerns the scanning phase of the aforesaid procedure. Scanning involves scanning the pictorial information latent in the imaging plate point by point and converting it into digital picture signals. The plate is generally scanned by sweeping a laser over it line by line. Mechanical irregularities can lead to scanning error, and it is occasionally necessary to test the scanner. A lead "phantom" is usually employed to produce a phantom image on the plate through the cassette. The phantom image is then read and processed in the scanner.
One drawback to this method of testing is that the results depend not only on the digitization unit but also on certain other parameters that cannot be definitely identified or quantified, such as the quality of the phantom image, the quality and position of the imaging plate in the cassette, and the nature and position of the lead phantom in relation to the cassette.